The usual packaging films made of plastic today are film laminates comprising a variety of layers (depending on application and function), for example, polyolefins such as polyethylene (PE) or polypropylene (PP), often combined with polyethylene terephthalate (PET) as the outer printed layer. Usually layers of different plastics are combined. Laminates of plastic layers with other materials such as aluminum or paper are also conceivable. Packaging is essentially also always produced with printing that is visible on the outside. The printing is applied to a layer of the film laminate that is suitable for printing, for example, a layer of biaxially stretched polypropylene (BO-PP) or polyethylene terephthalate (BO-PET).
In most cases, an inline printing process such as the gravure printing process or the inline flexographic printing process is used as the printing process for high-quality packaging materials. In an inline printing process, the individual printing units are separated from one another and the film sheet to be printed passes through a dryer and multiple deflecting rollers, in order to lengthen the drying length, before each application of the next ink. With certain films, in particular PE films, however, this leads to problems with the register accuracy and/or to unacceptable print images. PE films and PE film laminates are therefore usually printed by flexographic printing processes in satellite configuration on so-called central printing cylinder machines. In such machines the film sheet to be printed is held between the individual printing units on a central cylinder and only then is it dried. Also, a possible intermediate drying after one application of ink is carried out on the central cylinder wherein the film sheet is also held on the central cylinder during drying, but this does not usually allow complete drying between the printing units because the drying length is very short. Therefore, the print quality in a flexographic printing process in satellite configuration is not as high as it would be in the inline printing process. But the thereby achievable register accuracy is high enough even when using PE films, for example for diaper films.
With high-quality packaging laminates, the packaging manufacturers usually require printing of the film laminate by an inline printing process, for example by a gravure printing process or by a (UV) flexographic inline printing process, because of the print image which is thereby achievable. Therefore, a PET or PP film sheet is used as the printed film sheet in such film laminates, which are then laminated with a sealable material that is capable of sealing at low temperatures, such as PE films, to form the film laminate.
The film laminates for the packaging industry should naturally be as thin as possible for cost reasons. This means that the individual film layers should be designed to be as thin as possible, depending on their function. An outer layer which carries only the print image should be as thin as possible. According to the state of the art today, for example, printed PET film sheets with layer thicknesses of only 12 μm are used.
However, such film laminates that are used in the packaging industry are difficult to recycle because of the different materials they contain because the materials are difficult to separate. Therefore, there is a discernible trend to use so-called monolaminates in the packaging industry, i.e., film laminates made of (essentially) only one material. In particular, a monolaminate of polyethylene (PE) is of interest here because polyethylene is the most widely used sealing medium in the packaging industry. Film laminates consisting of a main material, for example, PE and containing only small amounts of any other material are also used as monolaminates. Such film laminates are still regarded as monolaminates in the sense of the present teaching and are relatively easy to recycle.
The problem with a monolaminate of polyethylene is that PE films with an industrially relevant thickness of less than 40 μm can be printed only to a limited extent or not at all in the required quality when using the inline printing systems, and in particular not by gravure printing process or a flexographic inline printing process. Thus, it has not yet been possible to produce such polyethylene monolaminates that are printed by the gravure printing process and can be used for the packaging industry.
DE 10 2005 003 922 A1 discloses a film laminate of a stretched HDPE printing film and an LDPE backing film, wherein the printed film is printed. The backing film should be significantly thicker than the printing film.